The present invention relates to a molding method or an ethylene/tetrafluoroethylene type copolymer (hereinafter referred to also as ETFE) powder. Particularly, it relates to a molding method for an ETFE powder whereby a coating film excellent in the surface smoothness can be obtained, and an ETFE powder therefor.
ETFE is excellent in heat resistance and chemical resistance and is therefore widely used as a material suitable for corrosion resistant lining or corrosion resistant containers, etc. As a processing method for corrosion resistant lining, there is a method wherein ETFE is powdered and subjected to a known method such as electrostatic powder coating, fluid bed coating or rotational molding to form coating films on various substrates. Further, as a processing method for corrosion resistant containers, there is a method wherein: ETFE is powdered and subjected to a mold, whereby a bottle molded product having any shape is obtained by a molding method such as rotational molding.
However, in a molding method for a powder, it sometimes tends to be difficult to increase the smoothness of the surface of the coating film, since no external shearing stress is exerted during the melting and solidification of a resin.
The present invention has an object to provide a molding method for a novel ETFE powder, whereby the surface smoothness of the coating film thereby obtained, can be remarkably improved. The present invention provides an ETFE powder which is made of ETFE of ethylene, tetrafluoroethylene and a fluorine-containing vinyl monomer copolymerizable therewith, said copolymer having a viscoelastic property represented by the formula (1) wherein the viscosity factor xcfx86 in the formula (1) is at most 1xc3x97104 Paxc2x7sec, and the anelasticity factor xcex5 represented by the formula (2) as the sum of xcex5i in the formula (1) is at most 5xc3x9710xe2x88x924 Paxe2x88x921, and which has an average particle diameter of from 5 to 1,000 xcexcm and an apparent density of at least 0.5 g/cc:                               J          ⁡                      (            t            )                          =                              t            /            φ                    +                                    ∑                              i                =                1                            6                        ⁢                          xe2x80x83                        ⁢                          [                                                ϵ                  i                                ⁡                                  (                                      1                    -                                          exp                      ⁡                                              (                                                                              -                            t                                                    /                                                      τ                            i                                                                          )                                                                              )                                                                                        (1)                                ϵ        =                              ∑                          i              =              1                        6                    ⁢                      xe2x80x83                    ⁢                      ϵ            i                                              (2)            
provided that in the formulae (1) and (2), J(t) is the amount of strain per unit stress under a predetermined stress, xcfx86 is a viscosity factor, t is time, xcex5 is an anelasticity factor, i is an integer of from 1 to 6, and xcfx84i is a constant (xcfx841=0.135 sec, xcfx842=0.368 sec, xcfx843=1.000 sec, xcfx844=2,718 sec, xcfx845=7.389 sec, xcfx846=20.086 sec)
Further, it provides a molding method for an ETFE powder characterized by depositing the above ETFE powder on a substrate surface, followed by heat melting and cooling for solidification to form a coating film.
ETFE to be used in the present invention is one obtained by copolymerizing tetrafluoroethylene (hereinafter referred to as TFE), ethylene and a fluorine-containing vinyl monomer copolymerizable therewith, wherein the molar ratio of polymer units based on TFE/polymer units based on ethylene is preferably 90 to 50/10 to 50. If the molar ratio of polymer units based on TFE/polymer units based on ethylene is lower than 50/50, the decomposition temperature tends to be low, whereby the moldability tends to be impaired. On the other hand, if this molar ratio exceeds 90/10, by-product polytetrafluoroethylene is likely to be present in ETFE, and the formed coating film is likely to form gels, or the high surface smoothness intended by the present invention, tends to be impaired.
The fluorine-containing vinyl monomer to be used in the present invention is a fluorine-containing vinyl monomer copolymerizable with TFE and ethylene, and the following ones may, for example, be mentioned. The fluorine-containing vinyl monomers may be used alone or in combination as a mixture of two or more of them.
Poly (or mono) fluoroethylenes such as CF2xe2x95x90CFCl, and CF2xe2x95x90CH2.
Poly (or mono) fluoropropylenes such as CF2xe2x95x90CFCF3, and CF2xe2x95x90CHCF3.
(Poly (or mono) fluoroalkyl) ethylenes such as CH2xe2x95x90CH(CF2)2F, and CH2xe2x95x90CH(CF2)4F.
Poly (or mono) fluoroalkyl fluoroethylenes wherein the poly (or mono) fluoroalkyl group has from 2 to 10 carbon atoms, such as CH2xe2x95x90CF(CF2)4F, and CH2xe2x95x90CF(CF2)3H.
Perfluorovinyl ethers such as CF2xe2x95x90CFO(CF2CFXO)mRf (wherein Rf is a C1-6 perfluoroalkyl group, X is a fluorine atom or a trifluoromethyl group, and m is an integer of from 0 to 5).
Vinyl ethers having a carboxyl group or a sulfonate group, such as CF2xe2x95x90CFO(CF2)3COOCH3, and CF2xe2x95x90CFOCF2CF (CF3) OCF2CF2SO2F.
The proportion of polymer units based on the fluorine-containing vinyl monomer is usually from 1 to 10 mols, preferably from 2 to 7 mols, per 100 mols of the total amount of polymer units based on TFE and polymer units based on ethylene.
For ETFE in the present invention, it is important that the viscosity factor xcfx86 defined by the above formula (1) is at most 1xc3x97104 Paxc2x7sec, and the an elasticity factor xcex5 defined by the above formula (2) is at most 5xc3x9710xe2x88x924 Paxe2x88x921.
The above formula shows the relation between the strain and the time when a very small stress is exerted in a molten state of the polymer heated at a temperature of at least the melting point. If the molten liquid is a Newtonian fluid, the strain increases linearly with time. Namely, it is considered to be a fluid whereby xcex5=0 in the formula (2). Whereas, ETFE is an agglomerate of linear molecules, and the molten liquid will not take an ideal Newtonian fluid behavior, since the molecules are entangled, and it shows a viscous behavior and an elastic behavior simultaneously against a very small stress, whereby the strain-time curve becomes non-linear. Namely, it is considered to be a fluid whereby xcex5 greater than 0 in the formula (2). If such ETFE is melted and left to stand still, the surface of the molten liquid becomes smooth and flat by free movement of molecules and the own weight of the molten liquid itself, while it shows an elastic behavior as the linear molecules are entangled one another, which prevents the surface from becoming completely flat and smooth.
ETFE in the present invention has a viscosity factor xcfx86 of at most 1xc3x97104 Paxc2x7sec, preferably from 500 to 7,000 Paxc2x7sec. When the viscosity factor xcfx86 is at most 1xc3x97104 Paxc2x7sec, mutual free movement of the molecules can be promoted. If the viscosity factor xcfx86 is larger than 1xc3x97104 Paxc2x7sec, free movement of molecules will be suppressed, and a large stress and time will be required to obtain a smooth surface, and it tends to be difficult to obtain a smooth surface.
Further, ETFE to be used in the present invention has an anelasticity factor xcex5 of at most 5xc3x9710xe2x88x924 Paxe2x88x921, preferably from 1xc3x9710xe2x88x925 to 1xc3x9710xe2x88x924 Paxe2x88x921. By adjusting the anelasticity factor xcex5 to be at most 5xc3x9710xe2x88x924 Paxe2x88x921, elastic resistance can be made small. If the anelasticity factor xcex5 is larger than 5xc3x9710xe2x88x924 Paxe2x88x921, the mutual entangling effect of molecules tends to be large, and it tends to show a more elastic behavior, whereby it tends to be difficult to obtain a smooth surface.
ETFE having a viscosity factor xcfx86 of at most 1xc3x97104 Paxc2x7sec can be obtained by controlling the molecular weight. If the molecular weight is too large, slipping of ETFE molecules one another will be suppressed, whereby the viscosity tends to be large, and the value of the viscosity factor xcfx86 will not be satisfied. Specifically, the control of the molecular weight of ETFE can be accomplished by e.g. the temperature, the pressure and/or the addition of a chain transfer agent during the polymerization. Further, as an index of the molecular weight of ETFE, the melt index (MI) is employed.
Further, ETFE having an anelasticity factor xcex5 of at most 5xc3x9710xe2x88x924 Paxe2x88x921 can be obtained by adjusting so that the proportion of polymer units based on the fluorine-containing vinyl monomer in ETFE will be usually within a range of from 1 to 10 mols per 100 mols of the total amount of polymer units based on TFE and polymer units based on ethylene.
Further, ETFE having an anelasticity factor xcex5 of at most 5xc3x9710xe2x88x924 Paxe2x88x921 can be obtained by charging the fluorine-containing vinyl monomer all at once at the initial stage of the polymerization, followed by the polymerization. The reason for this is not clearly understood, but is assumed to be such that the larger the number of polymer units based on the fluorine-containing vinyl monomer having a side chain, and the more uniform the distribution thereof in ETFE, the larger the mutual entangling effect of ETFE molecules, whereby the anelasticity factor xcex5 will be large.
Accordingly, it is considered that by carrying out the polymerization by charging the fluorine-containing vinyl monomer all at once at the initial stage of the polymerization, the proportion of polymer units based on the fluorine-containing vinyl monomer in ETFE formed at the initial stage of the polymerization, is large, and as the polymerization approaches the terminal stage, ETFE having a small proportion of polymer units based on the fluorine-containing vinyl monomer will be formed. Thus by making the distribution of polymer units of the fluorine-containing vinyl monomer to be non-uniform, it is possible to obtain ETFE having a suitable anelasticity factor xcex5.
The ETFE powder of the present invention is a powder having an average particle diameter of from 5 to 1,000 xcexcm, preferably from 10 to 700 xcexcm and an apparent density of at least 0.5 g/cc, preferably from 0.6 to 1.0 g/cc.
If the average particle diameter is smaller than 5 xcexcm, the powder particles are likely to form agglomerates having air internally occluded by an electrostatic action. If such a powder is melted and solidified, the coating film will be one having air occluded therein, whereby the appearance and the surface smoothness as an object of the present invention will be impaired. If the average particle diameter is larger than 1,000 xcexcm, it causes hard spots when it is melted and solidified, whereby the surface smoothness as the object of the present invention will be impaired. Further, if the apparent density is smaller than 0.5 g/cc, air present among the powder particles will be substantial, and when it is melted and solidified, it forms a coating film having air occluded, whereby the appearance and the surface smoothness as the object of the present invention will be impaired.
As a method for powdering ETFE, various conventional powdering methods may be employed. For example, there may be mentioned a method wherein ETFE is subjected to a mechanical pulverizer such as a pin mill or an impeller mill for powdering by an action of e.g. impact or shearing force, a method wherein a liquid having ETFE dispersed in a solvent, is sprayed in an atmosphere of at least the melting point for powdering, or a method wherein ETFE is subjected to a granulating machine such as a Henschel mixer, a high speed mixer or a mechanofusion for granulation and powdering.
The ETFE powder of the present invention has a feature that the melt flow property is high under a low shearing stress and is capable of realizing an extremely high surface smoothness even when melted and solidified under a static condition such as a powder molding.
In the molding method for the ETFE powder of the present invention, the ETFE powder is deposited on a substrate surface, followed by heat melting and cooling for solidification to form a coating film.
As a specific method for depositing the ETFE powder on the substrate surface, there may, for example, be mentioned a method wherein by an electrostatic powder coating machine, the powder is deposited on the substrate surface by an electrostatic action, a method wherein the powder is subjected to a fluidized tank and the powder is melt-deposited on a heated substrate surface, or a method wherein the powder is charged into a hollow substrate, which is subjected to a rotational molding machine for rotation to deposit the powder on the substrate surface.
The heat melting of the ETFE powder deposited on the substrate surface can be carried out by various conventional heating means. For example, heating in an electric furnace, heating in a gas burner furnace, heating by direct flame, or a continuous system electric furnace, may be mentioned. The temperature for heat melting may be a temperature where the ETFE powder will melt, and it is usually from 250 to 350xc2x0 C.
Further, the heat-melted ETFE powder is cooled for solidification, and the cooling for solidification may be carried out at a temperature where the heat-melted ETFE powder will be solidified, and it is usually carried out at a temperature of at most the melting point, specifically from room temperature to about 200xc2x0 C.
The thickness of the coating film formed by the ETFE powder is usually at least 30 xcexcm, preferably from 50 to 5,000 xcexcm.
The coating film to be formed by the present invention may be of a single layer or of multilayers.
A method for making the coating film to be of multilayers may, for example, be a method wherein on a substrate surface, at least one layer of a coating film layer formed by the ETFE powder used in the present invention or a layer formed by other ETFE, or a composite multilayer comprising at least one member from each of the two layers, is preliminarily coated, and the ETFE powder of the present invention is deposited thereon, followed by heat melting and cooling for solidification to form a coating film. The ETFE powder of the present invention has good compatibility with other ETFE in a molten state and is capable of forming a multilayer coating film which is scarcely peeled.
Other ETFE layer may be a coating film formed by depositing other ETFE powder on a substrate surface, followed by heat melting and cooling for solidification, or a coating film formed on a substrate surface by a coating material containing other ETFE, or having a molded product such as a sheet or a film of other ETFE laminated on a substrate surface.
Other ETFE is ETFE other than ETFE to be used in the present invention, and specifically, it is a copolymer of ethylene with TFE, or a copolymer of ethylene, TFE and a fluorine-containing vinyl monomer copolymerizable therewith, whereby the viscosity factor xcfx86 in the formula (1) exceeds 1xc3x97104 Paxc2x7sec, the anelasticity factor xcex5 of the formula (2) exceeds 5xc3x9710xe2x88x924 Paxe2x88x921, the average particle diameter is less then 5 xcexcm or more than 1,000 xcexcm, or the apparent density is less than 0.5 g/cc. The copolymerizable fluorine-containing vinyl monomer may be the same as described above.
The coating film to be formed by the present invention, may be made to have a surface smoothness of at most 0.1 xcexcm as measured by the surface roughness Ra.
The substrate to be used in the present invention is not particularly limited. For example, those made of various materials such as inorganic materials such as metals, concrete or ceramics or organic materials such as heat resistant plastics, may be mentioned.
Now, the present invention will be described in detail with reference to Examples (Examples 1 to 4) and Comparative Examples (Examples 5 and 6), but the present invention is not thereby limited. Further, in the following Examples, the physical properties and characteristics of ETFE, its powder and coating films, were measured by the following methods.
(1) Melt viscoelasticity: Using a Rotary rheometer (CS rheometer, manufactured by BOHLIN Company), the strain was measured under a load of 50 Pa at a temperature of 300xc2x0 C. with a plate diameter of 25 mm and a plate clearance of 1 mm, and by the curve fitting of the formula (1), the viscosity factor 4 and the anelasticity factor xcex5 were obtained.
(2) Melt index (MI): Using a Meltindexer (manufactured by Takara Kogyo K.K.), it was measured under a load of 5 kg at 297xc2x0 C. in accordance with ASTM D3159.
(3) Average particle size: Using an average diameter distribution measuring apparatus of a laser diffraction system (HELOS and RODOS, manufactured by Simpatec Company), it was measured by a dry method.
(4) Apparent density: Measured in accordance with JIS K6891.
(5) Film thickness: A magnetic substrate was measured by means of an electromagnetic system film thickness meter, and a non-magnetic substrate was measured by means of an eddy current system film thickness meter.
(6) Surface roughness: Measured by means of a contact system surface roughness meter.